1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly, the present invention relates to controlling a view angle which can be selectively driven between a wide view angle mode and a narrow view angle mode.
2. Discussion of the Related Art
Liquid crystal display (LCD) device controls electric field applied to liquid crystal cells. Thus LCD device displays pictures as incident light on a liquid crystal cell is changed by the electric field. LCD element is differentiated between a vertical electric field type and a horizontal electric field type according to an electric field direction which drives the liquid crystal.
In the vertical electric field type, a pixel electrode and a common electrode are formed on an upper substrate and a lower substrate which face each other vertically. The common electrode and pixel electrode are also formed to face each other in vertical direction thereon. The vertical electric field type applies electric field in vertical direction to a liquid crystal cell with a voltage applied to the electrodes. Twisted Nematic Mode (TN mode) is the most widely used LCD which is driven by the vertical electric field type.
In general, the development of the LCD focuses on improving an aperture ratio and ensuring a wide view angle. However, for works requiring security like using computers for personal purposes and doing bank related works, LCD with narrow view angle is needed. Therefore, it is an important task to develop a view angle controlling LCD in which the switching between the wide view angle mode and narrow view angle mode can be done easily.
In TN mode, currently the most widely used type, there are two ways of controlling the view angle: one way of using a viewing angle characteristic and the other way of using voltage-transmittance ratio characteristic according to the viewing angle.
FIGS. 1a to 1c illustrate the view angle controlling LCD using the voltage-transmittance ratio characteristic in TN mode.
Referring to FIG. 1, general voltage-transmittance ratio characteristic of the TN mode shows different curve according to the view angle. The curve shows that the voltage and transmittance are in inverse proportion when the driving voltage is within the boundary of 1V-2V to the view angle. However, the curve shows different voltage-transmittance ratio characteristic according to the view angle beyond the voltage boundary. In other words, when the driving voltage is more than 2V, the curve shows two different features. When the view angle is around 0°, certain transmittance ratio is maintained according to voltage increase. However, if the viewing angle is out of 0° range and increases, the transmittance ratio is increased according to the voltage increase. Using this voltage-transmittance ratio characteristic of the TN mode, the wide view angle mode and narrow view angle mode are selectively operated. The view angle controlling method of the TN mode using the voltage-transmittance ratio characteristic is to control the view angle by changing a driving circuit of the LCD so that the two driving voltage range is selectively output.
To operate the wide viewing angle mode, use the driving voltage range ‘A’ (about 1V˜2.6V). If the TN mode LCD is operated at the ‘A’ voltage range, decrease in the contrast ratio is small even though the view angle increases. Thus, left and right view angle and lower part view angle are wide as illustrated in FIG. 1B.
To operate the narrow viewing angle mode, use the driving voltage range ‘B’ (about 1V˜4.6V). If the TN mode LCD is operated at the ‘B’ voltage range, there is hardly a change in the contrast ratio when the viewing angle is 0° whereas decrease in the contrast ratio gets bigger according to increase in the view angle. Thus, left and right view angle and lower part view angle are narrow as illustrated in FIG. 1C.
In order to drive the view angle controlling LCD illustrated in FIG. 1A or FIG. 1C, it is necessary to output the liquid crystal operating voltage in two values at a DC/DC converter and input the two values to gamma power part and operate the LCD. However, the view angle controlling principle which has to design the voltage range differently has a very little effect as illustrated in FIGS. 1B and 1C.